The present invention provides a pin multiplexer including a multiplexing circuit, a control circuit and a detecting circuit. The multiplexing circuit includes a first port, a second port and a third port, wherein the first port, the second port and the third port are coupled to a first device, a second device and a third device, respectively. The control circuit is configured to control the multiplexing circuit to operate in a first mode or a second mode, wherein when the multiplexing circuit operates in the first mode, the first port is coupled the second port; and when the multiplexing circuit operates in the second mode, the first port is coupled to the third port. When operating in the second mode, the detecting circuit detects a signal of the first port to generate a detection result for dynamically switching the data transmission direction between the third device and the first device.